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Melatonin and other brain chemicals
One way that light influences your alertness is through melatonin, a hormone that induces drowsiness. Melatonin is produced in a predictable daily rhythm by the pineal gland, located in the middle of the brain, between the two hemispheres. Cells in the suprachiasmatic nucleus receive signals from the eyes about the amount of light in the environment, and send signals to the pineal gland that influence the gland's production of melatonin. Levels of melatonin begin climbing after dark and ebb after dawn. Scientists think that the daily light-sensitive cycles of melatonin help keep the sleep/wake cycle on track.
In addition to melatonin, other chemicals in the body influence our state of alertness. Several neurotransmitters (chemical messengers) play a role in arousal. Norepinephrine, epinephrine (often called adrenaline), and dopamine stimulate wakefulness. Adenosine and gamma-aminobutyric acid (GABA) are believed to promote sleep. Individuals vary greatly in their natural levels of neurotransmitters and in their sensitivity to these chemicals.
Because of these and other differences, people vary in their energy levels. Some people are naturally more energetic than others. Just because the person sitting next to you at work (or sitting across the table from you at breakfast) has more mental or physical energy than you do doesn't mean that there's something wrong with you. It could simply mean that you are at the lower end of the normal energy spectrum or that you're not a "morning person" and your alertness rises later in the day. But it's not normal to lack enough energy to get through the day. If you feel physically weak in the early part of the day, if you become fatigued soon after starting an activity, or if you are unmotivated or mentally weary much of the time, you need to find out why.
Aging and energy
It's a nearly universal lament of people over age 35: They don't have as much energy as they did when they were younger. One of the most common complaints is that they can't stay up as late as they once did and still function normally the next day. People also notice that they're not as strong as they used to be or that their muscles don't move as quickly.
Circadian changes
Although increased fatigue is not inevitable with increased age, there are certain age-related factors that make you feel weaker and, in general, less energetic. For one thing, your circadian cycle advances, making you want to fall asleep earlier at night and wake up earlier in the morning. Indeed, the most important influence on sleep rhythms is aging.
Insomnia becomes more common as people get older. For reasons that are not understood, older people spend less time in deep sleep, the type of sleep that is considered the most important for restoring your energy. With less deep sleep, you wake up more often in the middle of the night. And the more often you wake up at night, the less rested you feel the next day.
The amount of deep sleep that people get each night at age 30 is about half the amount they get at age 20. After age 30, the decrease in deep sleep and the increase in nighttime awakenings continue. Not only do people wake up more often in middle age, it takes longer to fall back to sleep. By age 65, people spend less than 5% of sleep time in deep sleep, compared with about 20% when they were in their 20s. In addition, melatonin levels decline with age and virtually disappear by old age. Because melatonin helps you feel sleepy at night, its decline can make it harder to fall asleep. The reductions in melatonin and in deep sleep help explain why insomnia becomes more common with age.
As you get older, you're more likely to make up for your nighttime sleep deficit by napping. But napping can make insomnia worse by keeping you from feeling tired enough at bedtime.
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Author Info: Harvard Health Publications
Date Last Reviewed: 11-01-2005 Published Date: 01-23-2007 Copyright © 2008 by President and Fellows of Harvard College. All rights reserved. |
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